The present invention relates to a chopped radiation source, preferably, a chopped optical radiation source which is useful in analytical devices such as photometers and spectrophotometers, particularly in photometers for monitoring chemical production processes.
Photometric and spectrophotometric devices for analyzing materials and monitoring reaction processes are known and commercially available. These devices compare radiation emitted by a source with radiation from that same source which has been transmitted through and altered by the sample material being evaluated.
One technique employed for improving the accuracy of measurements made with photometers and spectrophotometers is “chopping” of the radiation. In this technique, the radiation flow is interrupted at regular intervals or “chopped”. A chopped signal is necessary for AC coupling of amplifier stages in order to improve thermal stability. A chopped signal allows the use of electronic filters tuned to the chopped frequency to reduce background noise. Use of a chopper reference signal allows signal processing, sometimes referred to as lock-in amplification, to reduce background noise.
Electronic filters and a reference signal are employed, for example, in the signal comparison apparatus described in U.S. Pat. No. 2,678,581. In this disclosed apparatus, a beam of light is interrupted or pulsed at some predetermined frequency by a motor-driven rotating shutter or chopper wheel. A rotating sectored mirror is used to deflect the light pulses along one of two paths, and effectively acting as a second chopper at a lower pulse rate. Two fixed mirrors direct the light pulses toward a second rotating sectored mirror, in phase with the first rotating sectored mirror, to direct light toward a light sensitive element. Various properties of a sample, such as spectral characteristics or reflectance characteristics, may be compared by means of suitable devices positioned in the path(s) of the directed light pulses. The light pulse repetition rate is made substantially greater than the beam switching rate by providing a number of open sectors in the shutter and by using complementary half-round sectored mirrors. The light pulses received by the light sensitive element are converted to electrical pulses, which are capacitatively coupled between amplifier stages to eliminate temperature dependent DC drift. The signal is passed through electronic filters tuned to the chopper frequency to restrict bandwidth and reduce background noise. The signal is then processed by a lock-in amplifier that is mechanically synchronized with the chopping rate of the beam switching sectored mirrors. This results in decreased bandwidth, decreased noise and increased sensitivity.
One disadvantage of using a rotating chopper wheel such as that disclosed in U.S. Pat. No. 2,678,581 is the wear on the moving parts and chopper frequency stability. A tuning fork chopper, however, has no wearing movable parts and has a natural resonant frequency. For these reasons, a tuning fork chopper has been used in a number of deep space experiments where reliability is essential. Such a chopper is made in the USA by Electro-Optical Products Corporation and available from Boston Electronics Corporation. However, a tuning fork chopper is not suitable for use in all applications, particularly where there are space limitations. A vibrating reed chopper is used for those applications in which there are spatial constraints because it can be packaged in a smaller space than a tuning fork chopper.
Optical choppers have also been used in devices such as the infrared thermometer disclosed in U.S. Pat. No. 5,653,537 in which light emitted from an object is conveyed to a detector via fiber optic cable. The detector generates electrical outputs. These outputs or signals are then amplified and linearized. Unlike the prior art pyrometers, however, the pyrometers disclosed in this patent do not continuously collect and repeatedly chop the collected light before it enters the detectors. Rather, the collected infrared light is reflected off of a dichroic mirror and then passed through a piezoelectric chopper to chop the infrared light. Chopping is achieved through oscillation of a vibrating reed driven by piezoelectric elements. The chopped infrared light then enters a fiber optic cable and is relayed via that cable to an infrared detector which refocuses the light and then beamsplits the infrared light into two broad wavelength bands. One of these two broad wavelength bands is narrowed by an optical filter and detected by an infrared detector. The other broad wavelength band is directly detected by another infrared detector whose upper wavelength cutoff limits the wavelength band. The detectors generate an AC signal which is then amplified and linearized before being relayed to an analog to digital converter which generates the signal sent to a microprocessor.
In these and most other known photometric, spectrophotometric and pyrometric devices, alignment of the light source with the means for relaying that light to the sample and to the detector is one factor which influences the accuracy of the measurement and restricts the locations at which such devices must be placed to sites remote from the reactors in which chemical reactions are being conducted. The known devices also present problems with adjustment of the duty cycle of the chopping device. Use of moving parts subject to wear and tear also introduces the potential for inaccuracy and reliability problems.
It would therefore be advantageous to develop a chopped radiation source assembly which could be readily included in photometric, spectrophotometric and other types of analytical devices even when those devices are positioned in production areas without the need for realignment of light source with the chopper on a continuing basis or interruption of measurement for replacement of worn moving parts after a limited number of uses.